Prokaryte

Prokaryotes, comprising the domains Bacteria and Archaea, represent Earth's earliest and most diverse life forms. These single-celled organisms lack a nucleus and membrane-bound organelles, exhibiting unparalleled metabolic diversity crucial for global ecosystems. They are vital for human health as part of the microbiome but also include significant pathogens.

Life on Earth is classified into three domains: Eukarya, Bacteria, and Archaea. While Eukarya encompasses complex multicellular organisms, Bacteria and Archaea constitute the prokaryotes. These two domains together represent the two largest domains of life and account for the earliest forms of life on Earth. However, prokaryotes do not form a monophyletic group; evolutionary evidence suggests that Archaea are more closely related to Eukarya than to Bacteria.

Earth formed approximately 4.6 billion years ago, with the earliest chemical signatures of life appearing 3.8 billion years ago and the first fossil evidence (stromatolites) dating back 3.5 billion years. These early life forms were prokaryotic and adapted to a planet vastly different from today's. Stromatolites, layered structures formed by prokaryotic communities, diversified into distinct types by 3.1 billion years ago, showcasing early evolutionary and ecological diversification, including both photosynthetic and non-photosynthetic prokaryotes. Photosynthetic cyanobacteria, present at least by 2.5 billion years ago, began releasing oxygen into the atmosphere, leading to the 'oxygen revolution'.

Prokaryotes are characterized by their relatively simple, single-celled structure. They lack a nuclear membrane, meaning their genetic material is not enclosed within a nucleus, and generally have few to no membrane-bound organelles. Their genetic material typically consists of a single, circular DNA chromosome, though some may also contain smaller circular DNA pieces called plasmids. Prokaryotic cells are significantly smaller than eukaryotic cells, typically ranging from 0.5-5μm in diameter, about 100 times smaller by volume than eukaryotic cells. They reproduce asexually through a rapid process called binary fission, where a single cell replicates its DNA and divides into two daughter cells. Prokaryotes also exhibit metabolic diversity and the ability to evolve rapidly due to large population sizes.

Prokaryotic reproduction predominantly occurs via binary fission, a fast and asexual method. However, prokaryotes can acquire DNA from sources other than a parent cell, such as from other bacteria, the environment, or viruses, through a process known as horizontal gene transfer. This mechanism significantly contributes to their genetic diversity and evolutionary adaptation.

While both Bacteria and Archaea are prokaryotic, they differ in several key characteristics. For instance, Bacteria typically have one relatively simple RNA polymerase, whereas Archaea have a more complex RNA polymerase with more subunits, similar to eukaryotes. Archaea also share with eukaryotes the packaging of DNA with histone proteins and are considered more closely related to eukaryotes than to bacteria. These differences highlight the distinct evolutionary paths within prokaryotes and their relationships to the eukaryotic domain.

Prokaryotes are ubiquitous, found in virtually all environments on Earth. They demonstrate remarkable adaptation to diverse habitats, including symbiotic relationships with other organisms and extremophile lifestyles, thriving in conditions of extreme salinity, pH, pressure, or temperature. Their widespread presence underscores their foundational role in global ecosystems.

The human body hosts a complex community of prokaryotes, fungi, viruses, and protists known as the microbiome. The number of prokaryotic cells in and on our bodies is estimated to be greater than the number of human cells, accounting for about 70-90% of all cells, though they only make up about 0.3% of total body weight. This microbiome is crucial for human health, affecting digestion, protecting against disease, and sometimes causing illness.

Prokaryotes exhibit significant morphological diversity. Cell shapes include coccus (sphere), bacillus (rod), spirillium or spirochaete (spiral), and vibrio (comma-shaped). Cells can also arrange themselves in various patterns: diplo (pairs), strepto (chain-like), and staphylo (grape-like clusters). Another key morphological distinction is cell wall composition, determined by Gram staining. Gram-positive bacteria have a thick peptidoglycan cell wall and stain purple, while Gram-negative bacteria have a thin peptidoglycan layer surrounded by an outer phospholipid bilayer containing lipopolysaccharides and do not stain purple.

Metabolic diversity is a hallmark of prokaryotes, encompassing all four major nutritional strategies. Organisms require both ATP for energy and a carbon source (C-C bonds) for building biomass. Energy sources are phototrophs (light) or chemotrophs (chemicals), while carbon sources are autotrophs (fix CO2) or heterotrophs (absorb C-C compounds). This leads to photoautotrophs (e.g., cyanobacteria), chemoautotrophs (unique to certain prokaryotes, often extremophiles), photoheterotrophs (e.g., green and purple non-sulfur bacteria), and chemoheterotrophs (e.g., E. coli). Unlike eukaryotes, which only display chemoheterotrophic and photoautotrophic modes, prokaryotes utilize all four, reflecting their evolutionary adaptability to diverse environments.

Prokaryotes are vital members of ecosystems, performing numerous indispensable roles. Many are photosynthetic primary producers, like cyanobacteria, which generate oxygen as a byproduct and were responsible for the 'oxygen revolution.' Other photosynthetic prokaryotes, such as purple sulfur bacteria, produce sulfur instead of oxygen. Additionally, many prokaryotes are decomposers, breaking down dead organic matter and recycling nutrients. Certain prokaryotes can also fix atmospheric nitrogen (N2) into usable forms like ammonia, making it available for plants and other organisms, thereby playing a critical role in the nitrogen cycle.

Humans leverage prokaryotes for various practical applications. In molecular biology and biochemistry research, organisms like E. coli are widely used for genetic engineering, and the CRISPR-Cas system, naturally found in many bacteria, is a cornerstone of gene editing research. Prokaryotes are also essential in the food industry for producing dairy products and are employed in bioremediation to clean up pollutants like organic solvents from the environment.

Despite their beneficial roles, some prokaryotes are medically relevant pathogens. These bacteria can cause disease by producing toxins that damage host cells or alter their function, such as the lipopolysaccharides in the outer membrane of some Gram-negative bacteria (e.g., Salmonella). Many pathogenic bacteria can also form biofilms, which are colonies that coat surfaces and are common in nature. Biofilms pose significant challenges in medicine, as they can contaminate medical devices, contribute to conditions like tooth decay, evade the host immune system, and develop resistance to antibiotics, making them difficult to treat. The widespread evolution of bacterial resistance to antibiotics is a major public health concern.

Browse public study notes